The power supply circuits of a digital electronic computer have normal output voltage values which must be maintained within certain limits of variation if the data which the computer is processing is to be correctly reflected by the states of the various electronic circuits and magnetic devices that constitute the active circuits and the memory of the computer. A problem that sometimes arises is that an impermmissible variation has occurred in one or more of the power supply voltages, resulting in defective performance of one or more components of the computer, but the operator is not apprised of this condition.